This invention relates to meridian seeking instruments and, more particularly, to the calibration of such instruments.
Eklund U.S. Pat. No. 26,370 which issued Apr. 9, 1968, discloses a meridian seeking instrument that comprises a gyroscope unit having a horizontal spin axis and a container that encloses the gyroscope unit. The gyroscope unit hangs from a suspension band in the container, which is supported so it can rotate about a vertical axis. As the gyroscope unit rotates about the vertical axis under the influence of the earth's spin, the container is driven by a servo system so it follows the gyroscope unit, thereby reducing the twist in the suspension band. The container follow-up servo system comprises a transducer for generating a control signal representative of an angular displacement between the gyroscope unit and the container about the vertical axis, a motor for rotating the container, and a high gain amplifier for coupling the control signal to the input of the motor. The horizontal spin axis of the gyroscope unit oscillates symetrically about the meridian without appreciable damping. The true indication of the meridian is derived by bisecting the angle formed by the maximum excursions of the horizontal spin axis of the gyroscope unit as it oscillates.
The cross referenced Ambrosini patent discloses an arrangement in which the oscillations of the gyroscope unit about the vertical axis are damped by a torque force that is applied via a pair of coils to the gyroscope unit responsive to the previously mentioned control signal. Consequently, the horizontal spin axis of the gyroscope unit comes to rest in alignment with the meridian within a short period of time, e.g. five to ten minutes from initiation of gyro compassing. The torque force is applied to the gyroscope unit by a torquer comprising a first coil fixed to the container in approximately perpendicular relationship to the vertical axis and a second coil fixed to the gyroscope unit in approximately perpendicular relationship to the vertical axis and the first coil. The control signal or a secondary signal derived therefrom is applied to the first coil and a reference current is applied to the second coil. As a result, a torque force that is representative of the displacement between the gyroscope unit and the container is exerted on the second coil and transmitted thereby to the gyroscope unit to damp its oscillations.
The accuracy, or more properly, the repeatability of a meridian seeking gyroscope that hangs from a suspension band is impaired by the static error torques to which the gyroscope unit is subjected over a period of time. As used in this specification, the term "static error torques" refers to effects that change the angular position of the gyroscope unit about the vertical axis when its rotor is at rest. These effects act sufficiently slowly to be negligible in the course of taking a single fix, but are appreciable in the time interval between fixes. The sources of static error torques are diverse in nature. The effects of temperature changes on the suspension band is one of the major sources. Other sources are hysteresis effects and the permanent deformation of the suspension band that takes place as the suspension band ages. Stray magnetic fields also exert an influence on the position of the gyroscope unit. In the past, the effect of static error torques on the reading of the instrument has been reduced by providing an upper suspension band clamp that is rotatable. Prior to use of the instrument, it is calibrated by rotating the upper band clamp until the gyroscope unit assumes a null position about the vertical axis. This mechanical calibrating scheme is time consuming, leads to other errors, such as dislevelment of the instrument, and fails to attain an acceptable degree of repeatability from fix to fix. In addition, the provision of an external manual adjustment capability at the upper band clamp appreciably increases the size, complexity, and cost of the instrument.